Method, which one is correct? - switch mode power supplies

Thread Starter

recklessrog

Joined May 23, 2013
985
I have currently been running some posts on switch mode power supplies and how to work on them based on my many years of working in and owning a service company.
My ex business partner and I were debating method, like the question "which came first, the chicken or the egg",
Here is what I would like some opinions on, I present a little scenario and look forward to your replies.

Suppose you are asked to repair a simple a.m radio that is not working, you have it open on your bench, your equipment is a multimeter, an A.F signal generator, a R.F generator, and an oscilloscope.
Where do you start and how do you proceed? if possible, outline your reasons for doing it your chosen way.
This is for interest only, i'm just curious how others think :)
Lets have some fun with this.

(In the words of the Frank Sinatra song, "I did it MY way")
 
Last edited:

bertus

Joined Apr 5, 2008
21,141
Hello,

I would first check if the fault is in the LF or RF part.
The LF part can easely be checked using the LF generator, injecting some signal on the audio pot in the radio.
If that works, check the IF using a RF generator injecting signal on the IF transformer.
This can be a frequency between 455 Khz and 462 Khz, depending on the model.

Here also some interesting pages on repairing consumer electronica:
http://www.repairfaq.org/REPAIR/F_tshoot.html
http://www.repairfaq.org/REPAIR/

Bertus
 

MrChips

Joined Oct 2, 2009
22,874
You can start from the front, i.e. the antenna coil, or you can start from the end, i.e. the loudspeaker.
Either strategy is fine.

Generally, my debugging strategy begins before that:

1. Gather as much information on the history - When was the unit last working? What events led to the failure?
2. Look, smell, touch - thorough visual examination. Check the obvious - fuse, power switch, cord, broken connection, bad contacts.
3. Check the power supply.
 

paulgreat

Joined Jul 25, 2017
1
Well, base on my equipment (As listed above) i think i should test for the supply first it could be the device is not just receiving supply from the source. Then proceed to check the RF and LF part. Just from anywhere it's entirely up to you.

(DOING IT YOUR OWN WAY)
 

JUNELER

Joined Jul 13, 2015
183
Hi,
I thought you are more interested on smps power supply and what method is correct.
You did not mentioned your first method to the other method so we can cannot compare where you are.
I think chicken and egg cannot use for the method of comparison.
Please elaborate which subject in repairing you are more intense to work with
 

Thread Starter

recklessrog

Joined May 23, 2013
985
Hi,
I thought you are more interested on smps power supply and what method is correct.
You did not mentioned your first method to the other method so we can cannot compare where you are.
I think chicken and egg cannot use for the method of comparison.
Please elaborate which subject in repairing you are more intense to work with
This little thread is simply to see how other people on here would approach a minor repair. I think I can be mentally diverse enough to run more than one thread at a time :) The two threads are running concurrently.
Actually, in discussing the smp thread with my ex business partner who is still running the service centre I owned, we were surprised to discover that for years we had both been repairing the same items but sometimes in different ways, so by a little analysis of the approach used in the answers it get here, should enable me to give a broader cover of process by incorporating the thoughts and diverse methods used by other experienced members.
I had become increasingly concerned by the ideas some members had when dealing with any subject that related to live mains equipment, and as it is a taboo subject on the forum, I could see that the only option for the inexperienced, is to try out things that are potentially dangerous, with no guidance as to how to do it correctly and safely.
If the forum only dealt lets say with designs that required a supply maximum of 50 Volts, Then we would shielding the newbies starting out from valuable advice and disregarding probably 90% of how new equipment is powered. So where would the forum go from there? limit discussion to simulation only and become a Theoretical forum only?
Some of the members on here including myself, grew up from an early age working with thermionic valves (tubes) and the required high, sometimes very high voltages needed to power them. Hopefully not too many of us died in the process.
At the outset of the smp thread, I did point out that it is MY approach and not the ONLY way of going about it, but had proved to work well enough to enable me to become an O.A.P and once again enjoy electronics as a hobby.
 

AlbertHall

Joined Jun 4, 2014
10,896
Some of the members on here including myself, grew up from an early age working with thermionic valves (tubes) and the required high, sometimes very high voltages needed to power them. Hopefully not too many of us died in the process.
Yes, but I consider myself pretty lucky as I managed to connect myself, hand to hand, across the mains while a young teen. I still have the burn scars on my fingers. We do have some responsibility to protect the less knowledgeable from the voltages and currents available from the mains especially, as in my part of the world, it is 240V.
 

BR-549

Joined Sep 22, 2013
4,938
The first thing I would do is determine if there was any sound at the speaker. No sound...check power. Sound...inject low level audio signal or scope probe/finger to detector output. Listen for sound.

If you have power....the detector will tell you which way to go.
 

phranzdan

Joined Aug 4, 2017
40
I have currently been running some posts on switch mode power supplies and how to work on them based on my many years of working in and owning a service company.
My ex business partner and I were debating method, like the question "which came first, the chicken or the egg",
Here is what I would like some opinions on, I present a little scenario and look forward to your replies.

Suppose you are asked to repair a simple a.m radio that is not working, you have it open on your bench, your equipment is a multimeter, an A.F signal generator, a R.F generator, and an oscilloscope.
Where do you start and how do you proceed? if possible, outline your reasons for doing it your chosen way.
This is for interest only, i'm just curious how others think :)
Lets have some fun with this.

(In the words of the Frank Sinatra song, "I did it MY way")
 

phranzdan

Joined Aug 4, 2017
40
Hi: In my troubleshooting career, the first thing to check is the power supply. If battery powered check the terminal voltage. if ac powered check that the circuitry is actually getting power from the power supply circuitry.
 

Thread Starter

recklessrog

Joined May 23, 2013
985
Thanks for all the replies.
Here is how our system worked, we would have to repair around 300-400 items a week, to open the doors and run the business, costs including wages, building rent, phones, insurance, purchase of spares, office equipment, service equipment, everything down to toilet rolls, teabags, milk and coffee and a thousand other things, worked out at around 6k £'s a week! So, time was money.
Items would come in from trade contracts, the general public, service contracts etc. A Very important part of the procedure was booking in. Every item had a work sheet that had to be filled in with as much detail as possible such as, Symptoms of the fault, did it just fail to come on? was there any smoke, smell or sound when it failed? was it dropped, banged, allowed to be in a damp environment? Is the fault intermittent? and a few others. I had a to insist that the companies we where service centres for, also obtained this information and did not just send it in with a name address and "Dead" or suchlike appended to it. I made them very aware that there where financial penalties for failing to do this accurately, and that provided enough incentive to encourage them to comply.
Then, once said object was with the engineer, he had some clues as how to proceed.
Now the dead A.M radio example, read notes, worked yesterday, won't turn on today, no mention of being dropped or any of the other questions.
But people lie!! quick visual for signs of impact damage, turn on and listen closely at the speaker, any plop as power applied, any hiss at high volume? anything light up? this all done within one minute. so is it both mains and battery? check same again on both, open up, look for obvious corroded batteries, broken wires blown fuses etc two mins max. check for power, if mains only, is it reaching the radio? many times, we would find that the fuse was missing in the mains plug because another member of the owners household had taken it for some reason. Ok, got power in, d.c present at smoothing capacitor. turn off for a moment, put meter on low ohms range across speaker, listen for crackle and check meter for continuity. We must have changed hundreds of faulty o/c speakers, especially in one sugary brand of equipment!!! (some of you will know exactly who I'm referring to. :)
Speaker ok? turn on vol to max touch hot end of vol control with screwdriver, should produce clicks/ hum from speaker. NO?, fault probably now limited to audio section. (sometimes a faulty headphone socket was to blame with another brand in particular, the soldered pads on the pcb fractured due to careless use by owner) Most radios used a very cheap audio Ic that would fail for no apparent reason, voltage to it o.k, signal in, nothing out, no burnt resistors, no bulging caps was enough to say, New i.c required.
This is where you have to again weigh up cost. Initial purchase price of radio £50.00. We will get (from manufacturer if warranty repair) max £15.00 for radio repairs. Time to remove/replace I.c 15 mins due to screws plugs, desolder/ refit and test. total time now realistically 30 mins all for £15.00? I don't think so! Answer now is "Item Beyond Economical Repair"!
I will continue scenario in another post soon. (need coffee and breakfast :)
 
Last edited:

jayanthd

Joined Jul 4, 2015
904
I would start from power supply to different sections and then starting from antenna I would work out towards speaker.

power supply (all sections)
antenna
RF
IF
AF
demodulator
pre amplifier
amplifier
power amplifier
filter
speaker
 

Thread Starter

recklessrog

Joined May 23, 2013
985
I would start from power supply to different sections and then starting from antenna I would work out towards speaker.

power supply (all sections)
antenna
RF
IF
AF
demodulator
pre amplifier
amplifier
power amplifier
filter
speaker
Yes that's fine, But, in reality, there is more likely to be a fault in the audio output stage or power supply than in the R.F sections. So once having checked for power, you will probably find the fault quicker by working from the speaker back to the antenna, unless the antenna is missing/broken. Out of every thousand such repairs, 999 faults where not in the R.F sections.
 

bwilliams60

Joined Nov 18, 2012
1,379
This is a great thread and it is nice to see so many sharing their experience. It is too bad there are not sticky threadsor something on doing things exactly like this. How many inquisitive minds follow this forum and have to piece together some way of repairing their own equipment. You guys have sort of laid out the foundation for going at a radio problem. But how do you inject a signal, where do you make these test points, how do you put a scope on this equipment and what are you looking for? A lot of hobbyists don't have time to go to school but they are doimg this on their own and risking their lives to do I because they dont know better. I know the first AM/FM stereo I did, it was a large learning curve and a lot of help from forums but I got it done and still here to tell about it. Thank you guys for all that you do on this forum. CHEERS!
 
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